Mixing refers to a unit operation which makes two or more materials achieve a certain uniformity of dispersion by mechanical means or fluid flow. The mixing process is often associated with reduction and expansion of fluid volume, as well as mass transfer between different fluids. Mixing processes include macro-mixing, meso-mixing and micro-mixing, which are on different scales, respectively. Macro-mixing considers the dispersion, recycle and mixing processes at scales up to the size of the apparatus, meso-mixing considers the rate of dispersion from feeding site to main part of the reactor while micro-mixing is associated specifically with small scales down to the molecular level. Rapid and complex reactions are often encountered in process industries such as the field of chemical engineering, pharmaceutical and energy. The yield and quality of the product in rapid reactions is much more dependent on the mixing performance of the multi-phase reagents. So it is significant to develop novel high-performance mixer.
Over the past years, as micromachining technology is increasingly perfecting, we have certain knowledge on micro- and meso-physical phenomenon and use it in production and life. In micro-fluidic systems, influence of surface force and viscous force is dominant while inertia force is weakened since the ratio of surface area to volume increases rapidly; moreover, the rate of flow is usually small and belongs to laminar with small Reynolds number. Under the condition of laminar flow, mixing intensification can be realized by means of enlarging the interface between different phases by lengthening or shearing the laminar flow or significantly reducing the thickness of fluids by diverting, i.e. splitting liquid flow into smaller liquid flows, which can be achieved by creating relative motion between the liquid flows or changing the configuration of the flow channel. Currently, the more common method is designing more complex sequential or parallel flow path system according to micro fluid mechanics, to improve the mixing efficiency and reduce the mixing time and space required. In general, micro-mixers can be classified as either active or passive ones. Active mixing means the methods needing for external energy induced mixing, including micro stiffing, pressure disturbance, acoustic perturbation, electric driving, magnetic driving, thermal driving and so on. Passive micro-mixers do not require any external energy source, and they rely mostly on diffusion or chaotic advection using particular channel geometry that creates vortex flow. Chinese Patent Application No. CN101234324 presented a sequential passive multi-layer micro-contactor. The mixing section of the contactor is composed of parallel micro channels. Chinese Patent Application No. CN101234324 proposed a serpentine capillary heating tube with constant temperature heating device. Complex geometry is designed to enhance the convection effect of the fluids in the passive micro-mixers. However, as the fluids flow, the light phase and heavy phase gradually separate from each other as a result of the centrifugal force in the serpentine tube, which is the disadvantage of the mixing performance.